Within hours of the explosions that tore through a Madrid commuter train on March 11, Spanish police discovered a sports bag containing an unexploded bomb in the wreckage of one of the train cars.
Left next to sheet-covered bodies in a pile of the victims' belongings, the bag provided evidence that implicated al Qaeda's involvement in the attacks.
Photographs obtained exclusively by ABCNEWS show what the Spanish police discovered inside the blue sports bag.
The terrorists had used the alarm clock function of a cell phone to set off the bomb but had mistakenly set it for 7:40 p.m. instead of 7:40 a.m.
"It's a handy, dandy timing device," said Mike White, former commander of the New York City bomb squad, noting that the clocks in cell phones are driven by a centralized satellite timing system so they are synchronized.
"They can be set off either simultaneously or 10 seconds after each one," White told ABCNEWS.
Ten other backpack bombs were carried aboard four trains and detonated on schedule, almost simultaneously. Nearly 200 people were killed and 1,450 were wounded.
The unexploded bomb contained about 22 pounds of a whitish-colored plastic explosive.
Authorities say some of the explosive was stolen from a small Spanish mining operation a few weeks earlier. As the terrorists returned to Madrid, they were stopped at a police checkpoint but let through. A large quantity of bolts and nails was also discovered in the sports bag, packed next to the explosive to act as shrapnel.
"The inclusion of shrapnel in any improvised explosive device shows the intent of that device was to kill and to mutilate human flesh," White said.
The cell phone and the fingerprints on it led Spanish police to a Madrid cell phone store and the store's owner, the suspected ringleader of the attack, Jamal Zougam.
Zougam had been under police surveillance since September 2001 as a suspected al Qaeda figure but never arrested.
Spanish authorities now believe that Zougam and the 17 other suspects arrested in Spain are part of a European al Qaeda network that remains very much alive today.
ABCNEWS' Christine Romo contributed to this report.